1. Field of the Invention
The present invention relates to a mobile communication system, and in particular to a transmission power control method in a mobile communication system employing a code division multiple access (CDMA) method.
2. Description of the Background Art
In mobile communication systems using a code division multiple access (CDMA) method, such as a cellular mobile communication system and a personal communication service system, a plurality of mobile stations transmit/receive a frame consisting of symbols expressing a digitalized voice or other data through a base station or cell site.
In general, a maximum transmission power of the mobile station (terminal) is set according to a cell with a greatest radius. However, the mobile station always moves in a given cell. Thus, when some mobile stations are located closely to the base station, the others may be not. The terminals located closely to the base station have strong signals. In case the terminals with the strong signals are not controlled, they may cause unnecessary interference with the other terminals in association with transmission frames.
To the contrary, when the signal is transmitted by using a excessively low transmission power, the data of the frame which have been transmitted cannot be recovered, thus losing the data. Accordingly, the base station or mobile station controls the frame transmission power to minimize fading or interference through a multiple path and to completely recover the data to be transmitted.
A function of properly setting the transmission power according to the signal quality during the communication is the transmission power control, which is obtained by carrying out a dynamic power control between the mobile station (terminal) and base station.
The transmission power control process between the mobile station and base station will now be described in detail with reference to the accompanying drawings.
FIG. 1 is a schematic block diagram illustrating the conventional mobile communication system employing the CDMA method. A public switched telephone network (PSTN) is a communication network for normal telephone users provided by a communication network provider. The PSTN forms a wire communication path with a mobile switching center (MSC) 11 switching a circuit between users and processing relayed calls in order for the mobile communication user to communicate with another mobile communication user or a normal wire network user by using the mobile communication service.
In addition, a base station controller (BSC) 12 controls the power by using an error rate of the frames which have been received, and thus generates a power control order so as for the mobile station 13 to transmit/receive the data through base station A 14A or B 14B by employing a proper volume of transmission power.
FIG. 2 is a block diagram partially illustrating a power control device of the base station in the conventional mobile communication system using the CDMA method.
A spread-spectrum signal which has been transmitted from the mobile station (not shown) is received by a receiving antenna 20 and inputted to an RF receiver 21. The RF receiver 21 performs a down-frequency process on the inputted signal, converts the signal to a baseband signal, and outputs the converted signal.
The signal which has been down-frequency processed by the RF receiver 21 is converted to a digital signal in an A/D converter 22. A pseudo noise (PN) correlator 23 correlates the digital signal by using a PN code provided by a PN code generator (not shown).
Then, the signal which has been outputted from the PN correlator 23 is inputted to a decoder 24 and to a power measuring unit 25 for measuring the transmission power of the mobile station (not shown) which has transmitted the signal.
The decoder 24 decodes a code symbol regarding the inputted signal, and at the same time provides an outer loop power controller 26 with a code error metrics indicating a quality of the CDMA signal.
Accordingly, the outer loop power controller 26 is informed of an amount of transmission power required for the mobile station (not shown) to transmit the data, sets a power control reference value Po to be used for controlling the power of a reverse direction link, and provides it to a comparator 27.
The comparator 27 compares a transmission power level Pm of the mobile station (not shown) measured by the power measuring unit 25 with the power control reference level Po provided by the outer loop power controller 26, computes a power deviation Pd between the measured transmission power value Pm and the power control reference value Po, and provides it to a power control bit generator 28.
The power control bit generator 28 applies to a transmitter 29 an one-bit power control bit increasing or decreasing the transmission power than a current level by 1 dB according to the inputted power deviation Pd.
The transmitter 29 transmits the one-bit power increase or decrease bit outputted from the power control bit generator 28 to the mobile station (not shown) through a transmitting antenna 30 with a user data which has been up-frequency processed.
The mobile station (not shown) adjusts the transmission power according to the power increase/decrease order from the base station, and transmits the data by using the proper amount of transmission power.
Here, the power control performed by the power measuring unit 25, comparator 27 and power control bit generator 28 is a closed loop power control. The base station predicts a receiving power according to a transmission speed of the signal transmitted from the mobile station, compares the predicted receiving power with the power control reference values Po which have been pre-setted in every base station, and transmits the proper power control order to the mobile station with the user data, thereby enabling the mobile station to transmit the signal by using the adjusted transmission power according the power control order.
When the closed loop power control is carried out, each mobile station sets different power control reference values Po according to various parameters, such as a mobile speed of the mobile station and a circumstantial state thereof. Here, the base station controller (BSC) suitably adjusts the power control reference values Po according to the frame error rate of the data received by the outer loop power controller 26. The above-described power control is an outer loop power control.
On the other hand, the power control bit generator 28 generates the power increase or decrease bit according to the power deviation Pd inputted from the comparator 27. An one-bit power control bit has been generally used for increasing or decreasing the current transmission power level by 1 dB.
That is to say, as illustrated in FIG. 3a, when the measured transmission power level Pm is greater than the power control reference value Po, it is repeatedly decreased by 1 dB. As shown in FIG. 3b, in the case that the measured transmission power level Pm is smaller than the power control reference level Po, it is repeatedly increased by 1 dB.
Accordingly, when the deviation between the measured power level Pm and the reference level Po is great, there is a disadvantage that it takes a longer time to control the measured level Pm to an adequate proper level.
When the terminal tries to access, an initially-transmitted power is determined by using the open loop power control method. The power to be transmitted is determined by the following expression by employing the power currently received by the terminal.
Mean output power(dBM) = - mean input power(dBM) - offsetpower + NOR_PWR 16*NOR_PWR_EXT + INIT_PWR
The mean input power indicates an average power which the terminal receives the signal transmitted from the corresponding base station. The offset power of 73 dBm is used for the cellular mobile communication system and the offset power of 76 dBm is employed for the PCS, which are determined by the field test. NOR_ PWR and INT_ PWR are system parameters determined by an access parameters message transmitted from the base station. NOR_ PWR_ EXT is determined by an extended handoff direction message.
Here, the offset power is generally averagely determined by considering a load of the cells resulting from the number of the users operating in the current cell. That is, the terminal cannot be informed of the number of the users operating in the current cell, and thus does not know the amount of interference in the cell. Therefore, the offset power used in the cellular system or PCS system is averagely determined.
In case the cell where the user intending to access belongs is in an excessive load state, or the base station cannot receive an access signal from the terminal due to several damping reasons, the terminal cannot receive an acknowledge signal from the base station communicating that the base station receives the access signal. Thus, the terminal transmits the access signal gradually increasing the power of an access probe until the base station receives the access signal.
Accordingly, if the offset power is set smaller, it is difficult for the base station to sense the access signal. In this case, if the terminal constantly increases the power of the access channel, it takes a longer time to access. Therefore, the initial transmission power during the access is generally set at a sufficiently high level for the base station to easily recognize the access signal.
However, the terminal tries to access by using identical power even when the terminal is located closely to the base station or the cell load is smaller, and thus the power is unnecessarily consumed, which results in life span reduction of a terminal battery.
In addition, the terminal accesses with an unnecessarily high power, and thus may cause interference with the other users, thereby reducing capacity of the entire system.
When the terminal succeeds in accessing, is provided with a channel from the base station, and transmits a data through a traffic channel, the terminal transmits the traffic channel by using an identical power level to when it tries to access through the access channel. Then, as described above, the terminal constantly decreases the power by 1 dB, and thus it takes a longer time to adjust the power level to the adequate one, which results in high power consumption.